Mesalamine (also known as 5-aminosalicylic acid) is used to lessen the signs and severity of ulcerative colitis (remission). It does not cure ulcerative colitis (a bowel disease), but mesalamine may decrease symptoms such as stomach pain, diarrhea, and rectal bleeding caused by irritation/swelling of the colon/rectum. Mesalamine is an aminosalicylate anti-inflammatory drug. It is believed to work by blocking the production of certain natural chemicals that may cause pain and swelling. After an attack has been treated, mesalamine is used to increase the amount of time between attacks of ulcerative colitis.